Microspring with conductive coating deposited on tip after release

ABSTRACT

Efficient methods are disclosed for fabricating spring structures in which a passive, conductive coating is deposited onto the spring structure after release. A release layer is deposited on a substrate and then a spring metal layer is formed thereon. A first mask is then used to etch the spring metal layer to form a spring metal finger. A second (release) mask is then deposited that defines a release window used to remove a portion of the release layer and release a free end of the spring metal finger. The second mask is also used as a mask during the subsequent directional deposition of a conductive coating material on the cantilevered tip of the finger. The second mask is then stripped, and the residual coating deposited thereon is lifted off. The resulting spring structure includes conductive coating on the upper surface and front edge of the finger tip.

RELATED APPLICATIONS

[0001] The present application is a divisional of commonly ownedco-pending U.S. patent application Ser. No. 09/904,370, “MICROSPRINGWITH CONDUCTIVE COATING DEPOSITED ON TIP AFTER RELEASE” filed Jul. 11,2001 by David K. Fork and Christopher L. Chua.

FIELD OF THE INVENTION

[0002] This invention generally relates to stress-engineered metalfilms, and more particularly to photo lithographically patternedmicro-spring structures formed from stress-engineered metal films.

BACKGROUND OF THE INVENTION

[0003] Photo lithographically patterned spring structures (sometimesreferred to as “microsprings”) have been developed, for example, toproduce low cost probe cards, and to provide electrical connectionsbetween integrated circuits. A typical microspring includes a springmetal finger having an anchor portion secured to a substrate, and a free(cantilevered) portion extending from the anchored portion over thesubstrate. The spring metal finger is formed from a stress-engineeredmetal film (i.e., a metal film fabricated such that its lower portionshave a higher internal compressive stress than its upper portions) thatis at least partially formed on a release material layer. The freeportion of the spring metal finger bends away from the substrate whenthe release material located under the free portion is etched away. Theinternal stress gradient is produced in the spring metal by layeringdifferent metals having the desired stress characteristics, or using asingle metal by altering the fabrication parameters. Such spring metalstructures may be used in probe cards, for electrically bondingintegrated circuits, circuit boards, and electrode arrays, and forproducing other devices such as inductors, variable capacitors, andactuated mirrors. For example, when utilized in a probe cardapplication, the tip of the free portion is brought into contact with acontact pad formed on an integrated circuit, and signals are passedbetween the integrated circuit and test equipment via the probe card(i.e., using the spring metal structure as a conductor). Other examplesof such spring structures are disclosed in U.S. Pat. No. 3,842,189(Southgate) and U.S. Pat. No. 5,613,861 (Smith).

[0004] The present inventors have observed that conventional springstructures develop contact resistances that are detrimental to signaltransmissions when the spring structures are used as conductors. Thespring metal (e.g., Mo, MoCr, NiZr) is typically chosen for its abilityto retain large amounts of internal stress. These materials typicallyoxidize in air, a phenomenon that can interfere with their ability tomake electrical contact, for example, with the contact pad of anintegrated circuit when used in a probe card. The spring metal materialscan also gall to the contact pad, which is typically aluminum. Once thegalled aluminum oxidizes, the contact resistance between the contact padand the spring metal structure increases. One proposed approach toreducing contact resistance is to passivate the spring metal beforeetching and release. However, the passivating material tends to resistbending of the spring metal finger after release, and provides minimalcoverage along the front edge at the tip, thereby allowing directcontact with the spring metal that can result in increased contactresistance.

[0005] What is needed is a spring metal structure that resists increasedcontact resistance by avoiding oxidation of the spring metal and/orgalling of a contact pad against which the spring metal structure ispressed.

SUMMARY OF THE INVENTION

[0006] The present invention is directed to efficient methods forfabricating microspring structures in which a conductive coating isdeposited on the tip of the free (i.e., cantilevered) portion of thespring metal finger using a directional deposition process after releasefrom an underlying substrate. By directing the spring coating depositionon the spring metal finger tip after release (i.e. after the finger isallowed to bend upward from the substrate due to internal stress), theconductive coating is reliably formed on the front edge and uppersurface of the spring metal finger tip without impeding the bendingprocess, thereby producing a low-cost spring structure with reducedcontact resistance when compared to non-coated spring structures, or tospring structures coated before release.

[0007] In accordance with the disclosed method, a conductive releaselayer is deposited on a substrate, and then a stress-engineered (spring)metal film is formed on the release material layer. A first mask is thenused to etch an elongated spring metal island from the metal film, butetching is stopped before the release layer is entirely removed toprevent undercutting that can cause premature release of the springmetal island. A release (second) mask is then deposited that defines arelease window exposing a portion of the spring metal island and therelease material layer surrounding this exposed portion. In accordancewith an aspect of the invention, the release window is formed with anoverhang that helps prevent overlapping of coating material, therebyfacilitating lift-off of the residual coating formed on the releasemask. Subsequent removal of the release material exposed by the releasemask causes the exposed portion of the spring metal island to bend awayfrom the substrate due to its internal stress, thereby becoming the freeportion of a spring metal finger (an anchored portion of the springmetal finger remains covered by the release mask). The release mask isthen used as a mask during the deposition of the conductive coating(e.g., a refractory noble metal such as Rhodium (Rh), Iridium (Ir),Rhenium (Re), Platinum (Pt), and Palladium (Pd)) on the tip and otherexposed portions of the spring metal finger. The overhanging releasemask structure prevents overlapping of the coating material tofacilitate lift-off of residual coating portions during the subsequentremoval of the release mask.

[0008] In another embodiment, the release mask, which is also usedduring the deposition process, is provided with a channel extending overthe anchored (i.e., non-released) portion of the spring metal finger,thereby facilitating the formation of conductive coating portions on theanchor portion of the spring metal finger to improve conductivity.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009] These and other features, aspects and advantages of the presentinvention will become better understood with regard to the followingdescription, appended claims, and accompanying drawings, where:

[0010]FIG. 1 is a plan view showing a spring structure according to afirst embodiment of the present invention;

[0011]FIG. 2 is a cross-sectional side view of the spring structuretaken along section line 2-2 of FIG. 1, and the spring structurecontacting a separate integrated circuit;

[0012]FIG. 3 is a cut-away perspective view of the spring structureshown in FIG. 1;

[0013] FIGS. 4(A) through 4(J) are cross-sectional side views showingfabrication steps associated with the production of the spring structureshown in FIG. 1;

[0014] FIGS. 5(A) and 5(B) are plan views showing the spring structureof FIG. 1 during selected fabrication steps;

[0015]FIG. 6 is a cut-away perspective view showing a spring structureaccording to a second embodiment of the present invention; and

[0016]FIG. 7 is a plan view showing a release mask utilized tofabrication the spring structure shown in FIG. 6.

DETAILED DESCRIPTION OF THE DRAWINGS

[0017]FIGS. 1, 2 and 3 show a spring structure 100 according to a firstembodiment of the present invention. FIG. 1 is a plan view of springstructure 100, FIG. 2 is a cross-sectional side view taken along sectionline 2-2 of FIG. 1, and FIG. 3 is a perspective view with a cut-awaysection indicated by section line 3-3 in FIG. 1.

[0018] Spring structure 100 generally includes a substrate 101, arelease material portion 110, and a spring metal finger 120. Substrate101 (e.g., glass) includes an optional conductor 105 that can takeseveral forms (i.e., a metal trace deposited on the substrate, as shown,or a buried conductor accessed by an opening in a passivation layer (notshown)). When present, conductor 105 may provide electrical connectionbetween electronic components of an integrated circuit and springstructure 100. Alternatively, if substrate 101 is printed circuit board,printed wiring board, silicon device, or interposer, then conductor 105may be an exposed portion of conducting material that is electricallyconnected to redistribution traces, through substrate vias, solderbumps, solder balls, mounted electrical components, integrated passivecomponents, or interconnect pads. Release material portion 110 is formedon an upper surface of substrate 101 such that it contacts conductor 105(if present). Spring metal finger 120 includes an anchor portion 122 anda free (i.e., cantilevered) portion 125. Anchor portion 122 is attachedto release material portion 110 (i.e., such that release materialportion 110 is located between anchor portion 122 and substrate 101).Free portion 125, which includes a tip 125-T, extends from anchorportion 122 over substrate 101, and includes an upper (first) surface126 and an opposing lower (second) surface 127 that define a thicknessT1 in the range of 0.1 and 10 microns (see FIG. 2). Note that edges 128of free portion 125 extend between upper surface 126 and lower surface127.

[0019] Similar to prior art spring structures, spring metal finger 120is etched from a stress-engineered metal film that is formed by DCmagnetron sputtering one or more metals using gas (e.g., Argon) pressurevariations in the sputter environment during film growth in accordancewith known techniques. By carefully selecting the metals and/orprocessing parameters, sputtered metal films can be used to form tightlycurved spring metal fingers, or very stiff spring metal fingers, but notboth simultaneously because increasing the film thickness (which isnecessary to increase stiffness) also increases the radius of theresulting spring metal finger. Further, the internal stress of thestress-engineered metal film cannot be increased arbitrarily because ofmaterial limitations.

[0020] In accordance with an aspect of the present invention,electrically conductive coating 130 is formed to minimize contactresistance between spring metal finger 120 and contact structure (e.g.,a contact pad 152 formed on a separate integrated circuit 150, as shownin FIG. 2). In one embodiment, conductive coating 130 includes one ormore of the refractory noble metals (e.g., Rhodium (Rh), Iridium (Ir),Rhenium (Re), Platinum (Pt), and Palladium (Pd)) having a thickness T2in the range of 0.1 and 3 microns (see FIG. 2). To minimize contactresistance, conductive coating 130 is deposited on free portion 125 ofspring metal finger 120 after free portion 125 is released (i.e., afterrelease material located under free portion 125 is removed, therebyallowing internal stress to bend free portion 125 away from substrate101). Because conductive coating 130 is formed after free portion 125 isreleased, conductive coating 130 is deposited on upper surface 126 andedges 128 of free portion 125, and in particular on front edge 128-Tlocated at tip 128. Accordingly, as shown in FIG. 2, spring structure100 contacts external integrated circuit 150 through conductive coating130, thereby preventing increased contact resistance due to oxidation ofspring metal finger 120 or galling of contact pad 152. Severaladditional benefits provided by conductive coating 130 are described inthe following paragraphs.

[0021] First, forming conductive coating 130 after release allows springstructure 100 to be relatively thick (and, therefore, stiff), therebyincreasing the spring force constant of spring structure 100 at a lowercost than non-coated spring structures, or spring structures havingconductive coating formed only on one side. As indicated in FIG. 2, thedeposition process increases a total thickness of free portion 125 bythe thickness T2 of conductive coating 130. As described below,conductive coating 130 is formed at very low cost because the basictwo-mask process utilized for making non-coated spring structures is notviolated (i.e., no additional masks are used to perform the coatingdeposition process).

[0022] Second, forming conductive coating 130 after release allowsspring structure 100 to be both tightly curved and relatively thick(and, therefore, stiff) at a lower cost than non-coated springstructures, or spring structures having conductive coating formed onlyon one side. As indicated in FIG. 2 and discussed above, the curvature Rof free portion 125 is partially determined by the thickness T1 of thestress-engineered metal film from which it is etched. In order togenerate a tightly curved spring structure, a relatively thin metal filmis required. According to the present invention, spring structure 100can be both tightly curved and relatively thick by forming spring metalfinger 120 from a thin stress-engineered metal film, and then forming arelatively thick of conductive coating 130.

[0023] Third, conductive coating 130 passivates spring metal finger 120,which is important because most springy metals, such asstress-engineered metal film, form surface oxides. Conductive coating130 also increases wear resistance and lubricity, and thereby resiststhe galling of an associated contact structure (e.g., contact pad 152,which is shown in FIG. 2). Conductive coating 130 can also provide acompression stop to limit spring compression. Moreover, conductivecoating 130 may be added to strengthen spring structure 100 by addingductility. Finally, conductive coating 130 may be added to blunt theradii of process features and defects that can arise on spring metalfinger 120. The above-mentioned benefits are not intended to beexhaustive.

[0024] Note that optional conductor 105 is included to provideelectrical coupling of spring structure 100 to an external electricalsystem (not shown). Note also that the electrical coupling betweenspring metal finger 120 and conductor 105 necessitates using anelectrically conductive release material to form release materialportion 110. However, electrical coupling can also be provided directlyto spring metal finger 120 by other structures (e.g., wire bonding, orpre-patterning the release layer), thereby allowing the use ofnon-conducting release materials. Further, the cost-to-thickness(stiffness) characteristics discussed above may also be beneficiallyexploited in applications in which spring metal finger 120 is not usedto conduct electric signals.

[0025] FIGS. 4(A) through 4(J) and FIGS. 5(A) and 5(B) illustrate amethod for fabricating spring structure 100 (described above).

[0026] Referring to FIG. 4(A), the fabrication method begins with theformation of a conductive release material layer 210 over a glass(silicon) substrate 101. In one embodiment, release material layer 210is formed from an electrically conductive material, and a portion 210Aof release material layer 210 contacts a conductor 105 that is exposedon the upper surface of substrate 101. In one embodiment, releasematerial layer 210 is Titanium (Ti) that is sputter deposited ontosubstrate 101 to a thickness of approximately 0.2 microns or greater.Titanium provides desirable characteristics as a conductive releasematerial layer due to its plasticity (i.e., its resistance to cracking)and its strong adhesion. Other release materials having the beneficialplastic characteristics of titanium may also be used. In otherembodiments, release material layer 210 includes another metal, such asCopper (Cu), Aluminum (Al), Nickel (Ni), Zirconium (Zr), or Cobalt (Co).Release material layer 210 may also be formed using heavily dopedsilicon (Si). Further, two or more release material layers can besequentially deposited to form a multi-layer structure. In yet anotherpossible embodiment, any of the above-mentioned release materials can besandwiched between two non-release material layers (i.e., materials thatare not removed during the spring metal release process, describedbelow). Alternatively, when it is not necessary to provide electricalconduction between the subsequently deposited spring metal layer and acontact pad (such as conductor 105), release material layer 210 can be anon-conducting material such as Silicon Nitride (SiN).

[0027]FIG. 4(B) shows a stress-engineered metal film 220 formed onrelease material layer 210 using known processing techniques such thatit includes internal stress variations in the growth direction. Forexample, in one embodiment, stress-engineered metal film 220 is formedsuch that its lowermost portions (i.e., adjacent to release materiallayer 210) has a higher internal compressive stress than its upperportions, thereby causing stress-engineered metal film 220 to haveinternal stress variations that cause a spring metal finger to bendupward away from substrate 101 (discussed below). Methods for generatingsuch internal stress variations in stress-engineered metal film 220 aretaught, for example, in U.S. Pat. No. 3,842,189 (depositing two metalshaving different internal stresses) and U.S. Pat. No. 5,613,861 (e.g.,single metal sputtered while varying process parameters), both of whichbeing incorporated herein by reference. In one embodiment, whichutilizes a 0.2 micron Ti release material layer, stress-engineered metalfilm 220 includes Molybdenum and Chromium (MoCr) sputter deposited to athickness of 1 micron. In other embodiments, a Mo spring metal layer canbe formed on SiN release material layers.

[0028] Note that when conductive release material is used,stress-engineered metal film 220 is separated from conductor 105 byportion 210A of release material layer 210. Accordingly, a separatemasking step utilized in conventional fabrication methods to form anopening in the release material is not required, thereby reducingfabrication costs. Instead, as discussed below, the present embodimentutilizes the conductivity of release material layer 210 to provideelectrical connection between conductor 105 and stress-engineered metalfilm 220.

[0029] Referring to FIGS. 4(C) and 5(A), elongated spring metal (first)masks 230 (e.g., photoresist) are then patterned over a selected portionof stress-engineered metal film 220. Note that each spring metal mask230 extends over an associated conductor 105 (if present), as shown inFIG. 5(A).

[0030] Next, as indicated in FIG. 4(D), exposed portions ofstress-engineered metal film 220 surrounding the spring metal mask 230are etched using one or more etchants 240 to form a spring metal island220-1. Note that this etching process is performed such that limitedetching is performed in portions 210B of release layer 210 that surroundspring metal island 220-1 such that at least a partial thickness ofrelease layer portion 210B remains on substrate 101 after this etchingstep. In one embodiment, the etching step may be performed using, forexample, a wet etching process to remove exposed portions ofstress-engineered metal film 220. This embodiment was successfullyperformed using cerric ammonium nitrate solution to remove a MoCr springmetal layer. In another embodiment, anisotropic dry etching is used toetch both stress-engineered metal film 220 and the upper surface ofrelease layer portion 210B. This embodiment may be performed, forexample, with Mo spring metal, and Si or Ti release layers. Mo, Si andTi all etch in reactive fluorine plasmas. An advantage of dry etchingthe spring metal film is that it facilitates finer features and sharpertipped spring metal fingers. Materials that do not etch in reactiveplasmas may still be etched anisotropically by physical ion etchingmethods, such as Argon ion milling. In yet another possible embodiment,the etching step can be performed using the electro-chemical etchingprocess described in IBM J. Res. Dev. Vol. 42, No. 5, page 655 (Sep. 5,1998), which is incorporated herein by reference. Many additionalprocess variations and material substitutions are therefore possible andthe examples given are not intended to be limiting.

[0031]FIG. 4(E) shows spring metal island 220-1 and release material 210after spring metal mask 230 (FIG. 4(D)) is removed. Note again thatelectrical connection between conductor 105 and spring metal island220-1 is provided through portion 210A of release material layer 210.

[0032] Referring to FIG. 4(F), release (second) mask 250 (e.g.,photoresist) is then formed on a first portion 220-1A of spring metalisland 220-1. Release mask 250 defines a release window RW, whichexposes a second portion 220-1B of spring metal island 220-1 andsurrounding portions 210B of release material layer 210. In oneembodiment, release mask 250 is intentionally formed with a negativesloped side wall (for example, using negative resist or image reversaltechniques) to facilitate the liftoff of the conductive coatingdeposited thereon, as discussed below.

[0033] Referring to FIG. 5(B), note that release mask 250 is formed suchthat each spring metal island 230(1) through 230(3) formed on substrate101 is exposed through a separate release window RW1 through RW3,respectively. Separate release windows are provided for each springstructure because the release window will also define the pattern of thesubsequently deposited conductive coating. That is, if two or morespring structures were exposed through the same window, then conductivecoating would be deposited on substrate 101 separating the two springstructures, which would require an additional etching mask to remove theresidual coating. Accordingly, by providing separate release windows foreach spring structure, the total number of required masks is minimized.Note, however, that separate release windows result in greater spacingbetween spring structures to facilitate the formation of the releasemask wall separating the release windows. That is, providing space forthese release mask walls implies in principle that the spring structuresneed to be further apart.

[0034]FIG. 5(B) also indicates an optional dice line DL that extendalong the resist mask walls located between spring metal island 230(3)and an adjacent group of spring structures (not shown). Typically,substrate 101 is diced between the formation of release window 250 andthe lifting of the springs (discussed below). According to the presentinvention, batch processing spring structures is facilitated in that allof the masking is done before dicing (i.e., because no further masks arerequired to deposit and/or etch the conductive coating), therebypermitting a substantial reduction in total production costs incomparison to production processes, for example, in which the lifted andcoated spring metal fingers are dipped into a masking material.

[0035] Referring to FIG. 4(G), a release etchant 260 (e.g., a bufferedoxide etch) is then use to selectively remove a portion of the releasematerial layer from beneath the exposed portion of the spring metalisland to form spring metal finger 120 (discussed above with referenceto FIGS. 1-3). Specifically, removal of the exposed release materialcauses free portion 125 to bend away from substrate 101 due to theinternal stress variations established during the formation of thespring metal film (discussed above). Note that anchor portion 122remains secured to substrate 101 by release material portion 110, whichis protected by release mask 250. Note also that when release materialportion 110 is formed from a conductive release material, the resultingspring structure is electrically coupled to conductor 105.

[0036]FIG. 4(H) depicts an optional step for in-situ milling or backsputtering 270 that prepares the metal surfaces of free portion 125 foroptimal adherence of the subsequently applied coating using knowntechniques.

[0037]FIG. 4(I) shows the deposition of conductive coating 130 on freeportion 125 after release. According to an aspect of the presentinvention, this deposition step is performed using a directionaldeposition source (e.g., using line-of-sight material flux from a smallarea source, such as electron beam evaporation or sputter depositionusing a material collimator). Note that by performing the deposition ofconductive coating 130 after free portion 125 is lifted, top surface 126and front edge 128-T located at tip 125-T of free portion 125 face intothe deposition to facilitate uniform coating. Further, because thedirectional deposition facilitates the formation of conductive coating130 on front edge 128-T of free portion 125 after it is lifted, bettercoverage of tip 125-T is achieved than if deposition is performed beforerelease. Further, deposition of conductive coating 130 before therelease process runs the risk that the deposited coating will overlaythe release layer, and prevent or impede the flow of release etchant 260under free portion 125 (see FIG. 4(G) for reference) of the springstructure.

[0038]FIG. 4(I) also shows the deposition of residual coating portions130-R1 on release mask 250, and the deposition of residual coatingportions 130-R2 on regions of substrate 101 that are exposed through therelease window (i.e., adjacent to released free portion 125). Referringback to FIG. 3, note that residual coating portions 130-R2, which arenot removed by subsequent removal of release mask 250, are not depositedon portions 101-S of substrate 101 that are shadowed by spring metalfinger 125. Note that the negative-sloped side wall of release mask 250produces an exposed edge 250-E between coating portion 130 and residualcoating portion 130-R1 that allows access of a solvent during releasemask etch (discussed below) to facilitate liftoff of residual coatingportion 130-R1.

[0039] Finally, FIG. 4(J) shows spring structure 100 during the removalof release mask 250 and residual coating portions 130-R1 (see FIG.4(I)). Referring briefly to FIG. 4(I), note that the negative-slopedside wall of release mask 250 produces an exposed edge 250-E betweencoating portion 130 and residual coating portion 130-R1. Returning toFIG. 4(J), this exposed edge allows access of a solvent 270 thatdissolves release mask using known techniques. For example, when therelease mask is image-reversed photoresist, acetone can be used assolvent 270. As the release mask is dissolved, residual coating portionsformed thereon are lifted off. If necessary, agitation may be used toaccelerate the lift-off process. Because these residual coating portionsare lifted off during mask removal, there is no need to identify aselective etch for the coating material, which is typically difficult toetch chemically. Note, however, that residual coating portions 130-R2remain on substrate 101 (see FIG. 3). However, because each springstructure 100 is formed in a separate release window (as shown in FIG.5(B)), these residual coating portions remain isolated from adjacentconductive structures.

[0040]FIG. 6 shows a spring structure 300 according to a secondembodiment of the present invention. Similar to spring structure 100(discussed above), spring structure 300 includes a release layer portion110 formed on a conductor 105, a spring metal finger 120 formed onrelease layer portion 110, and a conductive coating 330 formed on springmetal finger 120. However, spring structure 300 differs from springstructure 100 in that conductive coating 330 is formed on both freeportion 125 and anchored portion 122 of spring metal finger 120(referring to FIG. 3, conductive coating 130 only covers free portion125). Specifically, conductive coating 330 is formed on both sides offree portion 125, as described above, and is also formed on an uppersurface of anchored portion 122. As mentioned above, it is wellestablished that resilient springy metals such as MoCr exhibitrelatively high resistance in comparison to many forms of conductivecoating, such as Ni, Au and Cu. Accordingly, by extending conductivecoating 330 over anchored portion 122, currents passing between freeportion 125 and conductor 105 are subjected to less resistance than inspring structure 100 due to the presence of conductive coating 330 onanchor portion 122.

[0041]FIG. 7 is a plan view showing a release mask 450 utilized in thefabrication of spring structure 300 (FIG. 6). Release mask 450 issimilar to release mask 250 (shown in FIG. 5(B)), except that therelease window defined by release mask 450 exposes part of the anchoredportion of each spring metal island 220(1) through 220(3). For example,referring to spring metal island 220(1), release window 450 includes achannel 455 that extends over anchored portion 222. Note that channel455 overlaps the outer edge 229 of anchor portion 222 by an overlapwidth OL of 1 to 10 microns to prevent unintended release of anchorportion 222. Referring briefly to FIG. 6, this overlap produces a stepstructure shoulder 325 extending along the edge of anchor portion 122after release, deposition, and removal of the release mask.

[0042] Although the present invention has been described with respect tocertain specific embodiments, it will be clear to those skilled in theart that the inventive features of the present invention are applicableto other embodiments as well, all of which are intended to fall withinthe scope of the present invention.

1. A spring structure comprising: a substrate; a release materialportion located over the substrate; a spring metal finger having ananchor portion attached to the release material portion such that therelease material portion is located between the anchor portion and thesubstrate, the spring metal finger also having a free portion extendingover the substrate; and a conductive coating deposited on a front edgeof the free portion of the spring metal finger.
 2. The spring structureaccording to claim 1, wherein the conductive coating comprises at leastone refractory noble metal.
 3. The spring structure according to claim1, wherein the spring metal finger comprises at least one of Molybdenum(Mo), Chromium (Cr), and Nickel-Zirconium (NiZr), and wherein theconductive coating comprises at least one of Rh, Ir, Re, Pt, and Pd. 4.The spring structure according to claim 1, wherein the release materialportion is electrically conductive.
 5. The spring structure according toclaim 4, wherein the release material portion comprises at least onemetal selected from the group consisting of Ti, Cu, Al, Ni, Zr, and Co.6. The spring structure according to claim 4, wherein the releasematerial portion comprises heavily doped silicon.
 7. The springstructure according to claim 4, further comprising a conductor formed onthe substrate, wherein the spring metal finger is electrically connectedto the conductor via the release material portion.
 8. The springstructure according to claim 1, wherein the conductive coating includesa second portion formed on the anchor portion of the spring metalfinger.